The present invention relates to a projection type color television system and, more particularly, to a correction circuit employed for correcting color luminance or brightness of an image projected on a screen of a color television set.
Generally, the projection type color television system has three cathode ray tubes disposed side by side to each other and each of the tubes emits light beams of one of different primary colors, red, green and blue, onto a projecting screen through a lens assembly. The three different color light beams form a proper color picture when they are projected onto the projecting screen in such a manner that images of different color are superimposed one over the other without any displacement. For accomplishing complete and perfect overlapping of the three different color images, it is necessary to produce the three different color light beams from exactly the same place. However, since it is impossible for the three cathode ray tubes to occupy the same place at the same time, they are aligned side by side to minimize the displacement of the light source between the three different primary colors.
The U.S. Patent Application Ser. No. 749,281, filed on Dec. 10, 1976 now U.S. Pat. No. 4,087,835 (in which the inventor of the present invention is involved), and my copending U.S. Patent Application Ser. No. 839,597, filed on Dec. 5, 1977, both assigned to the same assignee of the present invention, discloses such an arrangement as shown in FIG. 1 of the accompanying drawing. In the arrangement shown in FIG. 1, glass faceplates 11, 12 and 13 of respective cathode ray tubes 1, 2 and 3 are positioned on an imaginary plane which is in parallel to the projecting screen S and principal planes 04, 05 and 06 of the three lens assemblies L1, L2 and L3 are also positioned in another imaginary plane which is in parallel relation to the projecting screen S. It is also disclosed that the screen S includes a plate member 7 made of transparent material such as acrylic resin, having one surface facing the cathode ray tubes and formed into a lenticular lens, and the other surface being mat finished for forming a diffuser surface.
With such optical arrangement described above where the lens characteristics of the lens assemblies L1, L2 and L3 are alike to each other, the images of different color can be, after having passed through the respective lens assemblies, magnified so equally that, when they are projected onto the screen, the images of different color geometrically match with each other. Thus, the projected images in three different colors on the projecting screen S are properly superimposed one above the other to reproduce a color picture having no disadvantageous color displacement in such a manner with the distance D between the neighboring tubes being comparatively large. Furthermore, since a first plane including the phosphor screens of the three cathode ray tubes, a second plane including principal planes of the lens assemblies and a third plane including the projecting screen face are in parallel to each other, the proper focusing planes for the different color light beams projected from the three cathode ray tubes coincide with each other on the third plane including the projecting screen face. Therefore, from this aspect, in the optical arrangement of U.S. Patent Application Ser. No. 749,281, a relatively highly fast lens can be employed for each of the lens assemblies. With such arrangement described above, the lens assemblies applied in the optical arrangement were able to accommodate as bright as f/stop of 2.0, whereas those applied in the other arrangements were able to accommodate f/stop of 2.8, in obtaining the same reasonable color picture perceived by the human eyes, under the conditions in which the magnification of the lens assemblies is six and the diagonal distance of the screen is 750 m/m with the same projecting distance. Therefore, the brightness of the projected color picture on the screen with the optical arrangement of the above described type is twice as bright as those of the other types.
Although the optical arrangement of the above described type is superior to those of other types from the points of brightness and matching of projected images of three different colors, there have been a problem associated with an irregularity in the established color picture projected on the screen when the three color images are to be mixed. According to the previously mentioned copending U.S. patent applications, such disadvantage is eliminated by presentation of a screen assembly which is so assembled as to diffuse the light beam horizontally in a great angle of diffusion relative to the direction of travel of the incoming light beam and to diffuse the light beam vertically in a small angle of diffusion relative to the direction of travel of the incoming light beam. In contrast to such presentation of screen assembly, the present invention employs a correction circuit for eliminating irregularity in the established color images.